The European DVB-T (Digital Video Broadcasting-Terrestrial) standard for digital terrestrial television (DTT) uses Coded Orthogonal Frequency Division Multiplexing (COFDM) of transmitted signals, which are therefore transmitted as OFDM symbols.
Received signals are sampled in a receiver, and accurate reception and demodulation of signals therefore requires accurate knowledge of the positions of the beginning and end of each OFDM symbol.
In particular, DVB-T COFDM signals include a cyclic prefix to each active symbol, which is repeated after a known and fixed time interval. These cyclic prefixes must be correctly removed before demodulation, or the demodulation performance can be seriously degraded.
The fact that the prefix in the COFDM signals is repeated can be used initially to find the position of the prefix, by calculating a running correlation between received portions which are received separated by the known time interval. A very high correlation will indicate the presence of a repeated portion. However, this does not allow correction for any changes in position caused by subsequent variations in sampling rate.